Reusing Soil In Cannabis Growing

There is nothing quite like a perfectly balanced soil blend, that has the proper combination of nutrients, aeration and drainage, moisture holding capacity and PH. Many potting soils are expensive, and intuitively it might seem wasteful to use potting soil once to then discard it in favor of brand new soil. Thus for growing feminized, autoflower or photoperiod cannabis reusing potting soil from grow to grow can be appropriate in certain situations.

Can You Reuse Potting Soil?

There are several situations where reusing potting soil isn’t appropriate. First, never reuse soil if you are having fertility or general plant health problems. For example, if you are constantly battling yellowing plants, nitrogen deficiency, PH imbalance, etc, toss the soil and start anew.  A common fertility challenge with growers who reuse potting soil is actually excess phosphorus. 

Many growers focus on ensuring ample phosphorus which contributes to healthy flower development, which is the ultimate goal, right? We want to end up with huge yields of beautiful sticky smelly flowers. However, continuing to fertilize heavily with phosphorus can cause buildup which can actually lead to phosphorus lockout. What phosphorus lockout means practically speaking, is that the plants are unable (“locked out”) to uptake a variety of other nutrients including calcium, copper, magnesium, iron and zinc.

If you have soil borne pests, especially root aphids, use fresh soil.  In a plant positive approach to IPM, we are providing our plants with everything they need to grow and defend against pests through healthy soil.  Therefore if you are noticing more pest activity than usual, it may be an indicator that your soil blend is depleted.  So whether you have known, identifiable problems, or if your plants continue to have new and difficult to identify problems, related to fertility, pests or just general plant health we recommend switching out your soil.

If you do choose to reuse your potting soil there are a variety of things to consider. First, we recommend removing the root ball and as much of the previous plant as is reasonably possible. After you have removed the root-ball mix up the soil so it is nice and fluffy. This is performing “tillage” by hand. Lots of people use a long mixer  (3”-12”) in diameter depending on your pot size) on a cordless drill to make this process easier and more thorough. These mixers are a specially designed drill bit, so they plug directly into the drill.

Soil Flushing & Re-fertilizing 

 If you’ve been running a considerable liquid nutrient program in your grow, you may want to flush your soil. Flushing means running an excess of water through the soil, and the goal here is to “flush” as much fertilizer out of the soil as possible. For flushing, we recommend allowing water to flow off of your soil or soil medium for ten (10) minutes. 

Many growers already do this step several times in the last week or two of flower to try and help remove the fertilizer taste from the flowers. It is common practice to stop fertilizing in the last 7-14 days of flower.  But because reusing soil can often mean that your growing medium is now at least partially depleted, we recommend adding back in solid fertilizers which could include compost and lime for PH adjustments, or just good old fashioned fertilizer.

Soil Sterilization

The majority of this article is written for a grower on a small scale or those growing cannabis at home. However commercial growers have long utilized another option for soil re-use: sterilization. Sterilization can be done chemically (which we do not recommend) or via solar or steam sterilization. Solar sterilization is where you use clear plastic to raise the temperature of your soil, essentially baking it. 

Target temperatures are between 100 F and 175F depending on what is in your soil. One challenge with soil solarization is the time required. Depending on the season, expected temperatures, and latitude it may take weeks to achieve required temperatures. Steam sterilization requires specialized equipment. This is offset by the fact that these machines can raise soil temperatures very quickly and higher than via solarization, which means the cycle for solarization takes hours instead of days or weeks.

Conclusion

Reusing potting soil or growing mediums can be a cost effective way of reducing costs and simplifying processes. However it has to be done with care and caution. We have seen both home growers and commercial growers reuse soil for several years consecutively, which amounts to 10+ grows.

 Making Your Own Soil Blends

Have you ever wanted to make your own potting soil instead of overpaying for the newest trend at the neighborhood hydro store?  For those curious, this is an overview of why you may, or may not want to consider trying to build a soil craft blend.

The Art of Crafting Your Own Potting Soil: A Comprehensive Guide

1. Understanding Your Needs:
   Every grower has unique requirements when it comes to their plants. Some prefer soil for its impact on terpene and cannabinoid profiles, while others choose it for its convenience and ease of use. Consider your desired plant size, mobility requirements, and growing conditions to determine if potting soil is the right choice for you.
2. The Downside of Commercial Potting Soils:
   Commercial potting soils vary in quality, with inconsistencies in pH balance, fertilizer claims, and possible infestations. You might encounter problems such as root aphids, fungus gnats, or the presence of heavy metals and toxins. Achieving consistently great commercial potting soil can be a challenge due to supply chain issues and shelf life limitations.
3. Taking Control: Creating Your Own Potting Soil:
   To ensure a reliable soil blend, many cannabis growers have embraced the DIY approach. However, it’s important to test your homemade concoction on a small scale to fine-tune the mix before using it extensively.
   • Drainage, Aeration, and Moisture Holding Capacity:
     Finding the right balance among these three factors is crucial. A mix that drains too quickly leads to insufficient water retention, while one that retains too much moisture promotes lazy roots and invites fungal growth. Consider incorporating materials like vermiculite and regular soil components with larger particle sizes, such as perlite and lava rock, to achieve optimal aeration and drainage. Compost can also contribute to both moisture retention and drainage while providing fertility and essential minerals.
   • Solid Fertilizers:
     Determining the appropriate solid fertilizers and their quantities depends on factors such as pot size and your liquid fertility program. If you have a well-established liquid fertilizer regimen, solid fertilizers become less critical. Some organic options like bat guano, feather meal, bone meal, blood meal, and fish meal are widely used. Favor nitrogen-rich options during the vegetative growth phase, and consider top dressing or fertigation to supply phosphorus during the transition to flowering. Alternatively, incorporate larger amounts of slow-release fertilizers to ensure sustained plant availability.

Crafting your own potting soil empowers you to tailor the mixture to your specific needs, ensuring consistent quality and avoiding the pitfalls of commercial options. By understanding the importance of drainage, aeration, moisture holding capacity, and solid fertilizers, you can create a thriving environment for your cannabis plants. Experiment with small samples, adapt your recipe based on plant responses, and enjoy the satisfaction of cultivating in a personalized potting soil blend that perfectly complements your grow.

Here’s a soil blend recipe that you could try with some guidelines for mixing:

Custom Soil Blend Ingredients

• 1 part peat moss or coco coir
• 1 part perlite or vermiculite
• 1 part compost
• 1/4 part organic worm castings (optional)
• 1/4 part finely screened aged compost (optional)

Custom Soil Blend Instructions

1. Start by gathering the necessary ingredients in the specified ratios. Adjust the quantities based on the number of seedlings you plan to grow.
2. In a clean container or mixing tray, combine one part peat moss or coco coir with one part perlite or vermiculite. This mixture will provide suitable aeration and water drainage for young seedlings.
3. Add one part compost to the mixture. Compost provides essential nutrients and beneficial microorganisms that help with seedling growth.
   • (Optional) If available, add 1/4 part organic worm castings to the mix. What are worm castings? They’re an organic fertilizer made by earth worms that are rich in nutrients and can further enhance the soil blend’s fertility.
   • (Optional) Additionally, you can add 1/4 part finely screened aged compost for extra organic matter and nutrient content.
4. Thoroughly mix all the components together until you achieve a uniform blend. It’s essential to ensure the ingredients are well incorporated.
5. Moisten the soil mixture slightly, ensuring it is damp but not overly wet. This will help create an ideal environment for the seedlings to establish their root systems.
6. Fill your seedling containers or trays with the prepared soil blend, leaving some space at the top for planting the seeds.
7. Plant your cannabis seeds at the recommended depth, typically 1/4 to 1/2 inch deep, and cover them lightly with the soil blend.
8. Place the seedlings in a warm, well-lit area with proper humidity and adequate airflow.

Ultimately your goal should be to end up with a potting soil thats fits your system, which includes your grow space/climate, pot size, type of plant you are growing (autoflower = smaller and full term and semi-full term = larger), so tailor your potting soil to your specific needs.